Rotary kilns have become an essential bulk solids processing tool throughout a number of industries; their ability to facilitate physical and chemical changes through carefully controlled thermal treatment is nearly limitless in application.
Because rotary kilns utilize high temperatures to initiate and carry out reactions, processors looking to employ them must often incorporate a rotary cooler following the kiln.
The inclusion of a rotary cooler after the kiln allows processors to bring down the material temperature more quickly, which may be necessary for a variety of reasons. What follows provides information on when a cooler is needed, and considerations in how the cooler should be designed to handle material coming from a kiln.
When is it Necessary to Cool Material After a Rotary Kiln?
Whether or not a cooling step is necessary following a rotary kiln depends on the process and what will happen to the material after it exits the kiln. There are many potential settings in which processors may either benefit from, or require a rotary cooler after their kiln:
When Product Requires Immediate Handling
The most common setting in which a cooler is required is when the product needs to be handled immediately after processing in the kiln. Material exiting a rotary kiln can range in temperature anywhere from 100⁰C to 1500⁰C, which is often too hot for subsequent handling and could risk damaging equipment. Cooling the material exiting the kiln mitigates the potential for hot material to damage downstream processing and handling equipment.
Rotary coolers are typical in high-capacity, continuous production lines where there is no time to wait for material to cool naturally and the line must keep moving.
When Product Will Be Bagged After Processing
Similarly, cooling is also necessary when the product will be bagged after processing. In addition to risking damage to bagging equipment, depending on the material, there may be other problems associated with bagging a hot product.
Hot material sealed in a bag has the potential to cause caking issues, damage product, melt the bags, and even promote mold or bacterial growth. In all cases, the result is product degradation, which can lead to product loss or devaluation.
By cooling material prior to bagging, product and packaging integrity is maintained.
When the Material Presents a Safety Risk
As with the potential to damage downstream handling equipment, a hot product can also pose a safety hazard to workers, representing a significant liability. By feeding material directly from the kiln to a cooler, the potential for employees to accidentally come into contact with hot material is mitigated.
When an Ongoing Reaction Needs to Be Stopped
Rotary kilns are highly effective at carrying out various chemical and physical reactions in bulk solids, but just because material exits the kiln does not mean that the reaction immediately stops. In some settings, it becomes necessary to bring down the temperature of the material quickly in order to stop any ongoing reaction. This is commonly the case with reduction-type reactions where cooling the material quickly helps ensure minimal reoxidation of the product.
What is the Best Configuration for a Rotary Cooler Following a Kiln?
Rotary coolers are perhaps the most commonly employed type of industrial cooler when it comes to processing bulk solids coming off of a rotary kiln; they can handle a high throughput, are highly reliable and robust, and simple in operation. They are also very customizable, making them applicable in a range of process settings.
Since rotary coolers are designed around the specific characteristics of the material to be cooled, however, the best configuration is highly dependent on the parameters of the intended application. Common design considerations typically center around:
Direct or Indirect Rotary Cooler?
The first aspect of cooler design to consider is whether the cooler will be of the direct or indirect configuration.
Direct Rotary Coolers
Most coolers are of the direct design, as this is the most efficient approach to cooling. For this reason, producers working with a direct kiln will almost always employ the direct style cooler. Direct coolers are also the configuration of choice in this setting because direct kilns tend to work with higher temperatures than indirect kilns, which demands a highly efficient approach to cooling.
Direct coolers are nearly identical to their direct rotary dryer counterparts, but do not utilize any products of combustion. Instead, chilled or ambient air is pushed through the drum to cool material. Flights, or material lifters, pick up the material and cascade it through the air stream to maximize the heat transfer between the material and chilled air.
3D rendering of a FEECO Direct Rotary Cooler
Indirect Rotary Coolers
While direct coolers are the most commonly employed design, they are not a fit for all bulk solids cooling applications; in some cases, an indirect cooler may be the best option.
Unlike direct coolers, which rely on direct contact between the material and air stream, indirect coolers prevent contact between the two, instead cooling material through contact with the drum shell, which is enrobed in a chilled water bath. The advantage to this type of cooler, often referred to as an indirect water deluge cooler, is that it allows an inert processing atmosphere to be maintained, which is a common requirement when processing a material in an indirect rotary kiln (also commonly referred to as a calciner).
An indirect cooler is also beneficial when the material would otherwise risk entrainment in an air stream; fine materials could become swept up in the air stream and pulled out through the exhaust gas system. By utilizing an indirect cooler, the potential for entrainment is avoided.
3D rendering of a FEECO Indirect Rotary Cooler
Separate or Combined Cooling?
Most often, the cooling step is implemented through the use of a separate rotary cooler. In utilizing a separate unit, the rotary kiln is typically mounted above the rotary cooler, with a stainless steel connecting chute going from the kiln’s discharge breeching to the cooler inlet, allowing material to drop directly from the kiln into the cooler without any additional handling or equipment (for especially high temperatures, the chute may be refractory lined).
While it is not common, in some special process settings where indirect kilns are employed, it may be desirable to incorporate a cooling section right into the rotary kiln itself instead of using a separate unit. This is commonly seen when only a small degree of cooling is required, such as going from 800°C down to 600°C.
In this setting, the length of the kiln would be extended and fans or a small water bath added to that portion of the kiln only to cool the material.
Counter-Current Cooling (Direct Coolers Only)
While direct rotary dryers and kilns allow for either co-current (parallel) or counter-current air flow, it’s important to note that direct coolers employed after a rotary kiln are always of the counter-current design; the hottest material enters the cooler at one end, while the chilled air enters from the opposite end.
The counter-current air flow is much more efficient in high-temperature applications, because in this mode of heat exchange there is the ability to maintain a uniform temperature difference between the hot product from the kiln and the cold ambient air.
Materials of Construction
Rotary coolers can be fabricated from various materials of construction, depending on the characteristics of the material being processed. Direct rotary coolers are typically made from carbon steel, but are often made of stainless steel for handling materials coming off of a kiln, as it is more tolerant to heat. Other specialty alloys are also available for extreme temperatures or situations where corrosion is a concern. Similarly, abrasion-resistant and explosion-bonded steel are also sometimes useful.
Since material enters the cooler at its hottest, but cools as it moves through the unit, depending on the temperatures employed, it may be sufficient to construct only the inlet section of the cooler from a different material, leaving the remainder constructed of carbon steel or stainless steel.
For material coming in at especially high temperatures, the feed and beginning portions of the cooler are typically refractory lined. This gives material a chance to cool slightly before it comes into contact with the drum shell. A water spray system can also be employed in the feed section in order to quench and cool the material so it does not exceed the shell’s maximum allowable temperature.
Conclusion
Rotary coolers are an important addition to many rotary kiln processes, allowing material to be handled or bagged without risk to employees or equipment. They are also a useful tool in stopping ongoing reactions to reach precise product specifications. The best configuration for a rotary cooler following a kiln is highly dependent upon the specifics of the application, with options in direct or indirect cooling, a separate or combined cooling unit, and materials of construction.
FEECO is the leading provider of custom rotary kilns and coolers. We can provide complete rotary kiln systems, including the rotary cooler. We also offer an extensive line of aftermarket services for inspecting, auditing, repairing, and maintaining both kilns and coolers. For more information on our equipment or services, contact us today!